Lift mechanism and strategies of a variable-area flapping wing

In biological world and human sports, changing windward area of flapping wing in a flapping period is a common method to gain high lift. However, studies on the underlying mechanism are still rare. In the present paper, we select the multibody dynamics model proposed by Toomey & Eldredge as a basic model for flexible flapping wing, and add retractable plates between the lead and trailing bodies to enable the change of windward area. First, an intermittent strategy designed to increase the lift of the flapping wings is proposed and thoroughly examined. Results show that this intermittent strategy can increase the lift for narrower gaps, but leads to a reduction for wider gaps due to collisions between the trailing body and newly generated vortices. Then a modified intermittent strategy is further introduced to solve this problem. In this strategy, the gaps are closed earlier to promote the earlier formation of vortices, thus preventing the collision between the trailing body and the vortex structures. The modified intermittent strategy leads to significant increase of lift, especially for d=0.18 and d=0.20 (d represents the width of the wing gap). For d=0.20, the lift coefficient is increased from 0.57 (the intermittent strategy) to 1.48 (the modified intermittent strategy). Finally, by dividing the lift into three phases, the mechanisms for the lift increase under this strategy are analyzed. The significant increase of lift by changing windward area is expected to inspire and encourage future investigations of variable-area flexible flapping wing.